Impedance meter having signal leveling apparatus



1, 1966 B. M. ouvER IMPEDANCE METER HAVING SIGNAL LEVELING APPARATUSOriginal Filed Jan. 50, 1962 A REFERENCE AMPLIFIER COMPARATOR SOURCEOSCILLATOR 39 AMPLIFIER METER INVENTOR BERNARD M. OLIVER ATTORNEY3,283,242 HMPEDANCE METER HAVING SIGNAL LEVELING APPARATUS Bernard M.Oliver, Los Altos Hills, Calif, assignor to li-leWlett-llackard Company,Pain Alto, (Ialif, a corporation of California Continuation ofapplication Ser. No. 169,865, Jan. 30, 1962. This application Dec. 8,1965, Ser. No. 520,019 Claims. (Cl. 324-57) This application is acontinuation of pending US. patent application Serial Number 169,865,filed January 30, 1962, and now abandoned, and entitled Impedance Meter.

This invention relates to an impedance measuring circuit which providesdirect indications of impedance at selected frequencies and whichobviates the need for making separate current and voltage measurementsand for removing the components to be tested from the surroundingcircuitry.

Certain impedance measuring techniques are widely used in circuittesting applications where it is desirable to know the impedance presentat a test node at a selected frequency. For example, it is frequentlydesirable to determine the impedance characteristics with frequency of acoupling circuit in a cascaded amplifier. One known method of measuringimpedance uses a voltmeter and a current-responsive meter inserted inthe circuit under test to obtain current and voltage readings atselected frequencies. The disadvantages inherent in this method includethe fact that the current may divide into many paths at the test node sothe only way to measure the current is with a floating meter in the testlead. Further one must calculate the impedance at each of the selectedfrequencies. Other known methods, such as the use of an impedancebridge, may require the isolation of the components to be tested. Suchmethods are slow and often yield inaccurate results because thecomponents are not tested under actual operating conditions. It is thusdesirable to have a device which directly reads the impedance to groundat any selected frequency simply when touched to the node under test.

Accordingly, it is an object of the present invention to provide animpedance measuring device which reads impedance directly upon merelytouching a probe to the test node.

It is another object of the present invention to provide an impedancemeasuring device which avoids having to make separate measurements ofload current and load voltage.

It is still another object of the present invention to provide animpedance measuring circuit which measures the component in situ.

In accordance with the illustrated embodiment of the present invention,a source of alternating current is connected to the load to be testedthrough a probe which includes a current transformer and means to sensethe applied voltage. The alternating current is applied through oneconductor of the probe to the load under test, which conductorconstitutes a primary winding of the current transformer. The secondarywinding of the current transformer is returned through a secondconductor of the probe to a circuit which controls the amplitude of thealternating current applied to the load. The voltage produced across theload is returned through a third conductor of the probe to a voltmetercalibrated to read directly in impedance.

Other and incidental objects of the present invention will be apparentfrom a reading of this specification and an inspection of theaccompanying drawing in which:

FIGURE 1 is a schematic diagram of the circuit of the present invention,

ited States Patent 0 Patented Nov. 1, 1966 ice FIGURE 2 is a perspectiveview of the measuring probe of the present invention, and

FIGURE 3 is a schematic diagram showing a modification of the measuringprobe of FIGURE 2.

Referring now to FIGURE 1 there is shown a probe 9 having probe terminal11 and ground terminal 13. Alternating current from oscillator 15 isapplied through coaxial cable 17 to the probe terminal 11. The currenttransformer 19 comprises a one-turn primary winding, conductor 21, andsecondary winding 23. This transformer provides a first output signalwhich is returned through coaxial cable 25 to the amplifier 27. Theoutput of amplifier 27 is applied to one input of comparator 31. Theoutput of reference source 29 is applied to the other input ofcomparator 31. The output of comparator 31 is applied through conductor33 to a control element in oscillator 15. Oscillator 15 typically istunable over a wide range of frequencies. The voltage appearing betweenterminals 11 and 13 of probe 9 is returned through coaxial cable 35 toamplifier 37, the output of which is applied to meter 39.

In operation, the load 12 to be tested is connected between probeterminal 11 and the ground or reference terminal 13. Alternating currentfrom oscillator 15 is applied to the load 12 through the coaxial cable17. A signal proportional to the current flowing in the load circuit isreturned to amplifier 27 through the coaxial cable 25. The output ofamplifier 27 is rectified and compared with the reference source 29 incomparator 31. The error signal obtained as the difference between thevoltages applied to the inputs of comparator 31 is used to control theamplitude of the alternating current provided by oscillator 15. Thecurrent applied to the load circuit is thus maintained substantiallyconstant at some predetermined value for each measuring range.

At the same time, the voltage appearing across the load under test isreturned to amplifier 37 through coaxial cable 35. The output ofamplifier 37 is applied to meter 39 which provides a direct indicationof the voltage across the load' under test. Since the current in theload under test is maintained substantially constant, the voltage acrossthe load is proportional to the impedance. Meter 39 is calibrateddirectly in impedance, and thus provides a direct indication of theimpedance of the load under test. The gains of amplifiers 27 and 37 areadjustable in steps to provide successive ranges of impedancemeasurements.

A dual result may be obtained by maintaining the voltage across the loadunder test substantially constant and by metering the load current. Thereading in this case is a direct indication of load admittance.Specifically, a measurement may be obtained in this manner by comparingthe rectified output of amplifier 37 with the reference source incomparator 31. The error signal obtained in this manner maintains thevoltage at the output of oscillator 15 substantially constant. Thedirect indication of admittance is obtained by metering the output ofamplifier 27, which output is proportional to load current. Sinceadmittance and impedance are reciprocally related, any discussion hereinof measurement of either quantity should be taken to be applicable tothe other.

Referring now to FIGURE 2, the meter probe 9 of FIGURE 1 is shown inperspective view. Current from the oscillator 15 is applied throughconductor 21 to the load 12. This conductor constitutes a one-turnprimary winding for the current transformer 19. If the secondary winding23 has N turns, then the signal returned through coaxial cable 25 is l/Ntimes the load current. The primary winding may include more than oneturn, especially in low frequency applications where high inductivecoupling is desired.

The voltage appearing across load 12 is obtained at connection 41 and isreturned through coaxial conductor 35 to a sensitive voltmeter circuit.By sampling both the load current and load voltage substantially at thepoint where the load under test is connected to the probe, the effectsof voltage drop along the probe cables, of distributed capacity, and ofother factors upon the accuracy of the measurement are materiallyreduced.

Referring now to FIGURE 3, there is shown a schematic diagram of acircuit modification of the probe shown in FIGURE 2. The modificationadds to the current transformer 19 a one-turn winding 42 connected to anadjustable capacitor 45. In general, this additional winding has anumber of turns equal to the number of turns in the primary winding.This circuit compensates for the current drawn by distributed capacity43 at high frequencies. The effect of the load current in conduct-or 21due to the distributed capacity 43 is canceled by the effect of thecurrent in the one-turn winding 42 due to capacitor 45. Anothermodification of the probe of FIGURE 2 is made by extending the outershield of cable 17 beyond the core of transformer 19. This modificationreduces the distributed capacity between the primary winding and thetransformer core. Of course, an opening in the shield is required forconnection 41. For a multi-turn primary winding, the outer shield of theconductor is threaded through the transformer core along with theconductor.

The present invention provides a direct indication of driving pointimpedance simply by touching the probe to the load under test. Inaddition, rapid measurements may :be made using the circuit of thepresent invention since the load under test need not be removed from thesurrounding circuitry. Further, high accuracy measurements may beobtained over a wide range of test frequencies since the load currentand voltage are measured at points electrically close to the load. Thismeasurement accuracy is maintained even at extremely high frequencies inthe circuit of the present invention by the use of a compensatingcircuit which eliminates the effects of distributed capacity upon theindication of impedance.

I claim:

1. Apparatus for measuring the impedance of a load, the circuitcomprising:

a probe including a pair of test terminals for receiving thereacross aload to be measured;

a separate source of alternating signal having a selected frequencywhich is independent of load connected thereto;

signal conducting means serially connecting said source and the testterminals of said probe for applying the alternating signal from saidsource to the load to be measured connected to said test terminal;

means within said probe coupled to one of said test terminals forproducing a first output in response only to the current flowing througha load to :be measured connected to the test terminals;

means within said probe coupled to said test terminals for producing asecond output in response to the voltage across said test terminals;

circuit means connected to said source and responsive to one of thefirst and second outputs for altering the amplitude of the alternatingsignal to maintain said one of the first and second outputs constant;and

indicating means connected to receive the other of the first and secondoutputs for producing an indication of the magnitude thereof.

2. Apparatus as in claim 1 wherein the current flowing through the loadto be measured connected to said test terminals includes the currentflowing in a load to be measured and the current flowing in thedistributed capacity of the test terminals; and

' means within said probe coupled to one of the test terminals andresponsive to current flowing in said signal conductive means isprovided to decrease the first output by an amount related to thecurrent flowing in the distributed capacity.

3. Apparatus as in claim 1 comprising:

a current transformer within said probe coupled to said signalconducting means adjacent one of said test terminals for producing saidfirst output in response only to the current flowing through the load tobe measured connected to said test terminals; and

means connected to said signal conducting means at a point thereonadjacent said one of the test terminals on the source side of saidcurrent transformer for producing said second output in response to thevoltage across said test terminals.

4. Apparatus as in claim 3 wherein the current flowing through the loadto be measured connected to said test terminals includes the currentflowing in a load to be measured and the current flowing in thedistributed capacity of the test terminals; and

there is provided a compensating winding on said current transformer anda capacitor serially connected with the compensating winding poled tooppose the flux produced by coupling to the signal conducting means. 5.Apparatus as in claim 3 comprising: a source of reference signal; meansconnected to receive the reference. signal and first output forproducing an error signal related to the combination of said referencesignal and the first output; means connected to apply said error signalto the source for altering the amplitude of the alternating signal tomaintain the load current substantially constant; and indicating meansconnected to receive the second output for providing an indication ofthe impedance of the load to be measured connected to said testterminals.

References Cited by the Examiner UNITED STATES PATENTS 2,728,048 12/1955Priedigkeit 324-57 2,762,971 9/1956 Parker 32457 2,793,292 5/1957 Wolff32457 2,793,343 5/1957 Wagner 324-57 2,839,723 6/1958 DeArmond 324573,049,666 8/1962 Anderson 324-57 WALTER L. CARLSON, Primary Examiner.

W, H. BUCKLER, E. E. KUBASIEWICZ,

Assistant Examiners.

1. APPARATUS FOR MEASURING THE IMPEDANCE OF A LOAD, THE CIRCUITCOMPRISING: A PROBE INCLUDING A PAIR OF TEST TERMINALS FOR RECEIVINGTHEREACROSS A LOAD TO BE MEASURED; A SEPARATE SOURCE OF ALTERNATINGSIGNAL HAVING A SELECTED FREQUENCY WHICH IS INDEPENDENT OF LOADCONNECTED THERETO; SIGNAL CONDUCTING MEANS SERIALLY CONNECTING SAIDSOURCE AND THE TEST TERMINALS OF SAID PROBE FOR APPLYING THE ALTERNATINGSIGANL FROM SAID SOURCE TO THE LOAD TO BE MEASURED CONNECTED TO SAIDTEST TERMINAL; MEANS WITHIN SAID PROBE COUPLED TO ONE OF SAID TESTTERMINALS FOR PRODUCING A FIRST OUTPUT IN RESPONE ONLY TO THE CURRENTFLOWING THROUGH A LOAD TO BE MEASURED CONNECTED TO THE TEST TERMINALS;MEANS WITHIN SAID PROBE COUPLED TO SAID TEST TERMINALS FOR PRODUCING ASECOND OUTPUT IN RESPONSE TO THE VOLTAGE ACROSS SAID TEST TERMINALS;CIRCUIT MEANS CONNECTED TO SAID SOURCE AND RESPONSIVE TO ONE OF THEFIRST AND SECOND OUTPUTS FOR ALTERING THE AMPLITUDE OF THE ALTERNATINGSIGNAL TO MAINTAIN SAID ONE OF THE FIRST AND SECOND OUTPUTS CONSTANT;AND INDICATING MEANS CONNECTED TO RECEIVE THE OTHER OF THE FIRST ANDSECOND OUTPUTS FOR PRODUCING AN INDICATION OF THE MAGNITUDE THEREOF.